A drive support device, which operates based on map data, supports a driving safety by providing information of a forward road included in the map data. The information of the forward road is information of a road to be used by a vehicle. The drive support device provides a speed adjustment service that automatically reduces a speed of the vehicle when the vehicle runs a curved section of a road. The speed adjustment service is one of typical services provided by the drive support device.
The above-described speed adjustment service is executed only in a highway that has no junction. Recently, the speed adjustment service is also required to be applied to local roads, which have junctions.
When using information of the local roads included in the map data, a data amount of the information of the local roads, which is to be transmitted to the drive support device, is huge. When the drive support device supports a driving of the vehicle based on the map data, the drive support device needs to read information of the local roads to be used. However, it is impossible to absolutely predict the local roads to be used by the vehicle. Thus, the drive support device needs to consider information of all of the local roads that have the junctions.
Usually, a controller area network (CAN) is used for a data communication within the vehicle. The CAN has a limit to a data amount, which is also known as a communication band. Thus, the data amount of the road information to be transmitted to the drive support device is required to be reduced. Herein, the drive support device is a device placed in the vehicle to provide a drive support service.
A necessary service provided by the drive support device changes based on circumstances of surrounding roads. Thus, a map data obtaining scope and a map data obtaining method, which is also known as an algorithm, need to be changed based on the necessary service provided by the drive support device and an amount of the map data. The change of the map data obtaining scope and the map data obtaining method is required to be performed within the communication band of the CAN.
JR 3488319 B2 discloses a drive support device which uses road information of a scheduled route, which is calculated by a navigation device. However, a driver does not always select the scheduled route. Thus, only the road information of the scheduled route is not sufficient to support the driving of the vehicle.